Pastes are used in thick film technology in order to produce electrical components, such as resistors, capacitor layers, and insulating glass layers between printed conductor structures, as well as coating or protective layers which isolate a circuit applied to a ceramic substrate from the surrounding environment, on ceramic substrates. The composition of known thick film pastes is described, for example, in xe2x80x9cHerbert Reichl; Hybridintegration: Technologie u. Entwurf von Dickschichtschaltungen [Hybrid Integration: Technology and Design of Thick Film Circuits]; Huthig Verlag, Heidelberg, 1988, pages 37 to 63xe2x80x9d. Thick film pastes are printed on a ceramic substrate in the screen printing method. Known pastes contain a mixture of small solid particles having a high softening point or high sintering temperature and an inorganic binding agent, for example a glass having a low sintering temperature, which are dispersed in a vaporizable organic binding agent. The solid particles typically have a diameter of around 0.1 to p3 xcexcm. The dispersions have the consistency of a paste and have a rheology which makes the paste suitable for screen printing. Organic binding agents contain different additives such as binding and Theological agents, which increase the printability of the paste. For printing resistors, metal oxides, which determine the resistor""s conductivity, are also added to the paste. After screen printing, the printed structures are fired in a sintering process, the organic components being completely evaporated out of the paste or pyrolized. At a temperature below the sintering temperature of the solid particles,but above the sintering temperature of the inorganic binding agent, the latter is softened and flows around the non-sintering solid particles. Finished electrical structures have a thermal expansion behavior after cooling that is different from that of the substrate. Therefore crack formation occurs frequently in resistor structures in the case of temperature variations, whereby the electrical properties are disadvantageously influenced. In the extreme case the resistor completely fails. Cracks in a printed coating layer or insulation layer lead to a disadvantageous penetration of harmful substances or a short circuit of the printed conductors.
With the paste according to the present invention for screen printing of electrical structures on ceramic substrates, the tensile strength of printed structures is increased by a factor of ten and the danger of crack formation or fracture of the structures is reduced. The probability of failure of the circuit created on the substrate can clearly be reduced hereby, which is particularly significant in those cases in which the substrate is subjected to very high temperature change stresses or strong accelerating forces in operation, as is the case, for example, in motor vehicle electronic devices. The fracture toughness and strength of the structures is increased by fibers, which form the high-sintering solid portion of the screen printing paste. The fibers embedded in the fired structures (resistors or insulating layers) in this case increase the strength of the structures formed. The fibers are made out of a material which does not sinter at the sintering temperature of the inorganic binding agent. For example, ceramic fibers or HT glass fibers having a high softening point, or high sintering temperature, may be used here. Preferably the length of the fibers amounts to at least five times their diameter. The fiber-containing paste has good printability in the screen printing process, so that structures can be applied to the substrate with the known screen printing method.